This invention relates to a circuit for receiving a standard time signal and producing standard time pulses.
Standard time broadcasting stations are used in many countries to provide accurate timing signals. For example, station JJY in Japan employs carrier frequencies of 2500 KHz, 5000 KHz, 10,000 KHz and 15,000 KHz and also generates a pulse train (called a Second signal) wherein the interval between pulses is precisely one second. The Second signal is modulated at 1600 Hz. If this pulse train of one second duration is precisely reproduced it may be utilized to correct the time displayed by a clock, etc. However, in the cities there are many disturbing radio wave sources and also the propagation of short waves is much affected by the time of day, weather, etc. Therefore the signal is not necessarily received at all times. In particular, in the former case, the time signal is apt to be confused with extraneous noise components and in this case the noise might be received as time signals. In the latter case, if the radio wave is weak, its S/N ratio is small and therefore is apt to be affected by noise. It is difficult in these cases to receive and use all of the standard time signals in order to drive a clock or to correct the time thereof.
If the operator of a receiver can select a special time during the day when such noise is relatively low, good reproduction may be possible. But, it is in fact impossible to reproduce a correct time signal automatically and constantly.
However, an erroneous output may be avoided if an output signal is generated only when a true JJY signal is received and conversely, if such a signal is not generated when receiving conditions are not satisfactory. With this method, an output signal may not be generated for a relatively long time. Nevertheless, in correcting the time of a clock having a monthly error within several seconds, such correction is possible if a correction signal is generated only once a day. It is apparent that this correction method is far better, then permitting an erroneous correction by an erroneous signal.
Therefore, it is an object of the present invention to provide a standard time signal generator which generates an output only when it is confirmed that a received signal is derived from the standard time wave.
In order to achieve this object, the standard time signal generator according to this invention is constructed to select a pulse train of a standard wave receiver output according to a specific condition, to compare the selected pulse train with a reference pulse train, and if the result of the comparison satisfies another specific condition, to generate an output signal on the basis of the time of the first pulse of the pulse train of the receiver output, considering that this first pulse is a signal from the standard wave.
The standard time signal generator according to this invention therefore comprises a receiver tuned to a standard time signal carrier to reproduce a time signal pulse train; a reference time pulse generator to generate a reference time pulse train when a first pulse is generated from the receiver, the duration of the reference time pulse being identical to that of the time signal pulse train; a replaced pulse generator to generate a pulse only when a next pulse is not generated from the receiver in a predetermined time interval after generation of the first pulse from the receiver and to generate a replaced time pulse train which corresponds to the time signal pulse; a coincident counter to count time coincidence between the reference pulse and the replaced time pulse from the replaced pulse generator and an output pulse generator which generates a pulse when the number of the coincident counter reaches a predetermined value within a limited time interval.
The standard time signal generator of this invention therefore does not generate a signal when the receiving condition is not in order. Thus, correction of a clock by an erroneous signal may be avoided completely. Further, in view of the relatively simple construction of it, the circuit may be provided in integrated form, and mounted in household digital clocks for time correction. Of course, the circuit of this invention may be used for any other clocks of higher class.